GEOMORPHIC HISTORY OF THE COASTAL DUNE COMPLEXES ALONG THE SOUTHEASTERN SHORE OF LAKE MICHIGAN

In contrast to dunes along the northeastern coast that are commonly perched on high glacial bluffs, the dune complexes along the southeastern shore of Lake Michigan usually occur on former lake plains only a few meters above current lake levels. Simple dune complexes, like those south of Holland or in Van Buren State Park, contain a single set of parabolic dunes followed (moving inland) by lower backdunes. Compound complexes, like those at Hoffmaster and Warren Dunes State Parks contain a linear coastal ridge followed by several generations of overlapping parabolic dunes followed by backdunes. The backdunes in both types of complexes formed along a broad zone, extending several kilometers inland, during the rise to and fall from the Nipissing high lake levels. In places there appear to be two generations of backdunes with the older having formed around 7000 YBP and the younger having stabilized around 4000 YBP. Paleosols formed in depositional lobes of large parabolic dunes are now found in coastal exposures in the simple dune complexes, implying that these have been areas of net shore line retreat. In these areas the large parabolic dunes are exposed to wave action during periods of high lake levels. As wave erosion exposed sand to the prevailing westerly winds, the parabolic dunes migrated eastward along with the shore line. In the compound dunes a linear coastal dune ridge protects the parabolic dunes from wave action. These ridges probably formed by the coalescence of the foredunes that can be observed forming along their base during periods of low lake levels. The trailing edges of many of large parabolic dunes in the compound complexes are connected to fragments of linear dune ridges inland of the modern ridge. This suggests that these dunes originated as blowouts in dune ridges. The occurrence of fragments of up to 2 ridges inland of the modern coastal ridge suggests that compound complexes form in areas of net shoreline progradation. The main period of growth and migration of parabolic dunes in both the simple and compound complexes occurred between roughly 4000 and 2000 YBP. This was followed by a period of widespread dune stability marked by the formation of a relatively well developed paleo-Inceptisol. A new period of dune mobility began roughly 1000 years ago and accelerated during the last 500 years.